The present invention relates to a power transfer scheme for controlling the distribution of drive torque between first and second or front and rear drivelines of a vehicle.
A number of different power transfer systems are currently utilized for directing power, i.e., drive torque, to the front and rear drivelines of a vehicle. xe2x80x9cOn-demandxe2x80x9d power transfer systems automatically direct power to the non-driven wheels of a vehicle when traction is lost at the driven wheels. The xe2x80x9con-demandxe2x80x9d feature is incorporated into the transfer case by providing a clutch assembly that is interactively associated with an electronic control system and a sensor arrangement arranged to detect the wheel speed difference between the driven and non-driven wheels. During normal road conditions, the clutch assembly is maintained in a non-actuated condition such that drive torque is only delivered to the driven wheels. Sensors are provided to monitor the speeds of the driven and non-driven wheels. The driven and non-driven wheel speeds are utilized to generate a wheel slip or low traction condition. The clutch assembly is actuated based upon the directly measured relative speeds of the driven and non-driven wheels to deliver drive torque xe2x80x9con-demandxe2x80x9d to the non-driven wheels.
One example of such an xe2x80x9con-demandxe2x80x9d power transfer system is disclosed in U.S. Pat. No. 5,323,871 to Wilson et al. wherein the electronically-controlled clutch assembly is operable for automatically controlling the amount of drive torque transferred to the non-driven wheels as a function of the wheel speed difference between the driven and non-driven wheels. Similarly, U.S. Pat. Nos. 5,407,024, 5,485,894 and 6,062,330 to Watson et al., and 5,980,415 to Showalter each teach systems where power transfer is controlled by monitoring the respective speeds of the both the driven and non-driven wheels. Specifically, in U.S. Pat. Nos. 5,407,024, 5,485,894 and 6,062,330, a pair of primary and secondary drive shaft speed sensors are provided for measuring the respective speeds of the driven and non-driven wheels. Torque transfer is affected when the speed of one drive shaft exceeds the speed of the other drive shaft by a predetermined value. In U.S. Pat. No. 5,980,415, the speed sensors used to control torque transfer include rear driveline speed sensors and front driveline speed sensors. Clutch engagement is increased until the speed difference between the front and rear drivelines is reduced below a predetermined value. The clutch current is reduced when the speed difference between the front and rear drive shafts is reduced below the predetermined value.
For the reasons discussed in detail below in the Summary of the Invention, there is a need for a torque transfer control scheme that represents an improvement over the prior art xe2x80x9con demandxe2x80x9d transfer schemes that rely upon the sensed speed of both the front and rear drivelines.
This need is met by the present invention wherein the present inventors have recognized that the aforementioned xe2x80x9con demandxe2x80x9d systems all rely upon the sensed speed of both the front and rear drivelines and, as such, present operational and design challenges that need to be addressed by an approved torque transfer scheme. From a design standpoint, it is difficult to minimize system cost if hardware and software must be provided for sensing the speed, and the differences in speeds, of both the driven and non-driven wheels.
Torque transfer schemes that are dependent upon the sensed speed of both the front and rear drivelines are typically deficient from an operational standpoint in that they are subject to the introduction of error through routine vehicle acceleration. More specifically, regarding the introduction of error through routine vehicle acceleration, routine acceleration causes a loss of traction in the driven wheels of the vehicle. This loss of traction generates higher wheel speed data and a corresponding distortion in the data representing the difference in speeds of the driven and non-driven wheels. This distortion, which varies depending upon the coefficient of friction of the road surface, must then be accounted for in the software or hardware of the torque transfer system.
Use of a signal indicative of the difference between the front and rear wheel speeds is also subject to the introduction of error under tight turning conditions because the front and rear wheels turn in separate turning circles. More specifically, the speed error increases with vehicle speed and steering angle so that when the front wheels reach a certain speed (e.g., 5 mph) during the tightest turn (e.g., at full steering wheel lock), the difference in speed between the front and rear wheels (e.g., 2 mph) is often sufficient to engage torque transfer. This type of false engagement is likely to occur every time a vehicle turns regardless of the coefficient of frictionxe2x80x94even if there is no loss of traction.
From an additional operational standpoint, it is difficult to address xe2x80x9cplowingxe2x80x9d conditions in torque transfer schemes that are dependent upon the sensed speed of both the front and rear drivelines. Specifically, plowing is a condition that occurs when the front, or non-driven, wheels of a vehicle enter deformable surfaces, such as standing water, mud, dirt, sand, snow or gravel. The weight of the vehicle causes the non-driven wheels to sink into the deformable surface and the material of the deformable surface (e.g., water, mud, dirt, sand, snow, or gravel) plows up against the front of the non-driven wheels, slowing them down. In effect, the non-driven wheels travel uphill, smoothing the surface for the rear, driven wheels. In most cases, the driven wheels are unaffected by this plowing condition because they will maintain the commanded speed. The present invention is based upon the recognition that the prior art scheme, where control of torque transfer is based upon the speed of the driven and non-driven wheels, will cause false engagement of the transfer case because the sensed speed of the non-driven front wheels is not accurate under plowing conditions. Specifically, in the prior art system, the difference between the front and rear wheel speeds could be skewed enough by the plowing to cause activation of the transfer case and transfer of torque to the non-driven wheels.
For example, if a truck maintains a true vehicle speed of 25 mph and crosses an unplowed intersection of six inches of snow, the non-driven front wheels may abruptly drop in speed down to 21 mph or less while the driven rear wheels maintain a speed of 25 mph. If the threshold for activating torque transfer is about 3 mph, the 4 mph difference between the front and rear wheels in this case will cause torque transfer to be affected even though there is actually zero wheel slippage.
In accordance with one embodiment of the present invention, a method of controlling torque transfer is provided. According to the method, a delta signal indicative of a rotational condition of the first driveline is generated and a torque transfer unit is controlled as a function of a torque transfer control algorithm. The control algorithm commands a transfer torque representing an amount of torque to transfer from the source of power to one of the first and second drivelines. The torque transfer control algorithm is a function of the delta signal and is substantially independent of any rotational condition of the second driveline over a primary operational range of the algorithm.
In accordance with another embodiment of the present invention, a method of controlling torque transfer is provided where a delta signal indicative of a difference between a reference value and a rotational condition of the first driveline is generated. The reference value comprises a desired vehicle acceleration value determined as a function of maximum output torque, a desired torque command signal generated by a throttle position sensor, vehicle weight, and grade. The rotational condition of the first driveline is indicative of a temporal change in one of rotational speed or rotational acceleration of the first driveline. The torque transfer unit is controlled as a function of a torque transfer control algorithm. The torque transfer control algorithm is a function of the delta signal and commands a transfer torque representing an amount of torque to transfer from said source of power to one of said first and second drivelines.
In accordance with yet another embodiment of the present invention, a method of controlling torque transfer is provided where a delta signal indicative of a rotational condition of the first driveline is generated. The delta signal comprises a signal indicative of a difference between a reference value and a selected one of a plurality of different rotational conditions of the first driveline, including a primary rotational condition and at least one alternative rotational condition. The reference value comprises one of a desired vehicle acceleration value, an actual vehicle acceleration value, a vehicle ground speed value, temporally displaced first driveline acceleration values, temporally displaced first driveline speed values, and combinations thereof. The torque transfer control algorithm determines the selected rotational condition. The algorithm exhibits a preference for the alternative rotational condition over the primary rotational condition when a difference between the reference value and the rotational condition does not exceed a torque transfer threshold and when a difference between the reference value and the alternative rotational condition exceeds the torque transfer threshold.
Additional embodiments, including additional methods of torque transfer, torque transfer units, and vehicles incorporating the same, are also contemplated by the present invention. Accordingly, it is an object of the present invention to provide improved methods and apparatus for torque transfer. Other objects of the present invention will be apparent in light of the description of the invention embodied herein.